1. Field of the Invention
The present invention relates to methods for making skylights, and more particularly to methods for forming skylights from polycarbonate plastic sheets.
2. Description of the Prior Art
In the past, a variety of skylights have been available which are typically formed from thin sheets of a transparent plastic polymer material. A sheet is clamped in a frame, heated, and vacuum formed to produce a dome. Such skylights may be attached either directly to a flat portion of a roof by a perimeter flange, or alternatively, attached to a curb formed in the roof. It has been common to utilize plastics such as the acrylics and butyrates for these prior art skylights. These types of plastic sheet used in the prior art skylights are noncritical with respect to forming and molding. For example, a sheet of acrylic plastic may be clamped in a clamping frame, the assembly heated by means of an oven or the like, and thereafter formed by molds or vacuum forming. There are few if any problems due to warping of flanges, or to rapid cooling of the plastic during the molding process. Apparently, the acrylic plastics have a relatively high heat retaining capability and will therefore remain soft and plastic over a sufficiently long time to permit completion of the molding operation. One major disadvantage of the acrylic and butyrate type skylights is a lack of mechanical strength which presents difficulty in meeting national building codes and the Underwriters Laboratory standards for burglary resistance.
Prior art apparatus and machines for vacuum forming of skylight dome elements are exemplified by U.S. Pat. No. 3,025,566 to Kostur in which an adjustable clamping frame is mounted in a carriage which permits a plastic sheet clamped in the frame to be moved between an upper and lower oven box for heating of the sheet. After heating, the carriage is moved back to its loading position and a set of platens closed on the sheet, vacuum applied to one side of the sheet and the desired shape thus vacuum formed. A rotary adjustable clamping frame is disclosed by Asenbauer in U.S. Pat. No. 3,599,959 which permits use of a loading station, two sequential heating stations and a vacuum forming station. A sheet of plastic may be loaded, in a clamp frame, rotated to the first heating station. While heating, a second sheet is loaded. After passing through the second heat station and the forming station, the finished product arrives at the loading station, is removed, and a flat sheet loaded. Thus, a continuous operation may be maintained.
Polycarbonate plastic sheet, as exemplified by Lexan.RTM. manufactured by General Electric, has characteristics ideally suited for use with skylights due to its superior strength and resistance to breakage. It has been found that skylights can be fabricated from polycarbonate sheet which will meet the national building codes and the burglary resistance standards of the Underwriters Laboratory. However, the above referenced apparatus are eminently suited for forming of acrylic and similar plastic sheets but were found unsatisfactory for forming skylight dome elements from polycarbonate plastic sheet. The use of such prior art techniques and machines resulted in a very low yield of satisfactory units. It has been determined that the problem stems from the relatively narrow range of temperatures over which polycarbonate sheet can be efficiently formed, and the more rapid cooling of the sheet when it is removed from the heating means. These problems show up as excessive rippling or waviness in flat perimeter flanges, and in distortion or irregularities in the dome portions. As may be recognized, polycarbonate skylight domes must be as optically clear and distortion free as possible, perimeter flanges must be flat to seal tightly against roof or curb surfaces, and, due to the higher cost of polycarbonate materials, be producible with a high yield.
The present invention is an improvement in the art of vacuum forming as applied to polycarbonate sheet in the heating and forming apparatus and methods.